1. Field of the Invention
The present invention relates to copiers, and more particularly, to apparatus and method for improving the contrast and density of copier copies.
2. Description of the Prior Art
The term contrast, as used herein, refers to the rate of change (or slope) of the output copy density Dout with respect to the input document density Din.
In one control technique (closed-loop), the charge on a photoconductor is measured and the contrast and density of copies adjusted by varying one or more of the copier process parameters. See commonly assigned U.S. Pat. No. 3,779,204, issued Dec. 18, 1973 and U.S. Pat. No. 4,087,171, issued May 2, 1978. Although this technique is useful, it is sometimes difficult to accurately measure the charge or voltage level on a photoconductor.
A copier "open-loop" control technique is more frequently used. In this technique, the copier can selectively be operated at one of three modes of operation which correspond to normal, lighten, or darken copies. Normal, lighten, or darken copies generally refers to copy density relative to input document density. For example, when a lighten copy is designated, copies are made lighter than original documents. The open-loop technique depends on the stability of all the copier parts to maintain a consistently high-quality output. Since the response of the copier parts is not always identical, let alone prefectly stable, some compromise is made during setup to allow variations to occur without causing objectionable defects in copies. For example, as developer ages, it causes changes in copy density. Consequently, in the set-up for normal copy operation, exposure is usually set intentionally light (overexposed) to prevent any possible appearance of background as the developer ages. In this situation, line copy and solid area density are often not optimal. Due to manufacturing tolerances, different batches of toner and developer may have different contrasts. The term toning contrast is defined later in this specification. Suffice it here to say, since most copiers operate with fixed parameters, copy density may vary with toner or developer batches having different toning contrasts and occasionally be objectionably low or high.
With a given concentration of a particular toner in a developer, there are three copier operating levels that usually are varied to change copy contrast and density. They are the voltage applied onto the photoconductor by the primary charger, the copier exposure, and the bias voltage applied to the development station. The bias voltage has the same polarity as the electrostatic image and effects toner deposition. Varying the bias voltage provides some control of copy contrast and density (especially minimum copy density).
These three levels are interrelated and their proper adjustment by an operator would require considerable skill and judgement on his part. Consequently, copier manufacturers have elected to provide an operator with only limited control over copy contrast and density.
Commonly assigned U.S. patent application Ser. No. 137,149, entitled Copy Contrast and Density Control, filed Apr. 4, 1979 to Fiske et al, discloses apparatus which produces copies having improved copy contrast and density. As disclosed therein, a memory has a stored matrix array of sets, with each such set having values which correspond to specific levels of Vo, Eo, and V.sub.B respectively. The operator designates a particular set. Means responsive to the values of the designated set change the exposure Eo produced by exposure lamps, the voltage Vo applied onto the surface of a photoconductor by a charger and the bias V.sub.B applied to an electrode of a development station to provide a copy having improved line and solid area contrast and density. The values of a given set cause the copier to produce copies with a desired Din/Dout response curve for a "manufacturing standard" copier. If a copier is non-standard, due to manufacturing variance in copier parts and toner, the values of a designated set will not produce a copy in accordance with the desired Din/Dout response curve.
To overcome this problem, as disclosed in the above-identified copending application, a larger matrix array is stored in ROM than is needed for a standard copier. For example, if the array size needed for a standard copier is 9.times.9, then the larger array size, which includes the smaller array, may be 15.times.15. If, in such a scheme, a desired Din/Dout response curve for a normal copy of a standard copier is at matrix array position (5,6), its corresponding position for a non-standard copier must be found within the larger matrix array. This position may, for example, be at position (5,7). The contiguous 9.times.9 array positions in the 15.times.15 array are then used until a recalibration is performed. Although this arrangement has performed satisfactorily, it requires an increase in memory space. Also in certain situations, copy contrast and density still may not be in accordance with a desired Din/Dout response curve.